Effect of rhizobia from caucasian clover(Trifolium ambiguum)on nodulation and nitrogen fixation of white clover(Trifolium repens)

Abstract

Three glasshouse trials investigated N 2 fixation in white clover following inoculation with rhizobia from caucasian clover. We grew white clover in agar, vermiculite/perlite and soils low in available-N and with no detectable rhizobia. Media were subsequently treated with inoculum containing rhizobia effective on either (i) caucasian clover, (ii) white clover, or (iii) mixtures of the two. The percentage of ineffective nodules on white clover increased as the proportion of rhizobia from caucasian clover in the inoculant mix increased. However, the percentages of ineffective nodules on white clover were less than the proportion of ineffective rhizobia in the inoculant, indicating that the ineffective rhizobia were less competitive in nodule formation. Foliage yield and symbiotic N 2 fixation in white clover were not significantly affected when ineffective rhizobia from caucasian clover made up 67% or less of the inoculant. A03019; Online publication date 26 February 2004 Received 3 April 2003; accepted 15 August 2003However, when ineffective rhizobia made up a high proportion of the inoculant mix (>92%), both foliage yield and N 2 fixation were significantly reduced compared with plants inoculated with only white clover rhizobia. Extrapolation of the present glasshouse results to the field is difficult and further field trials are required to determine if rhizobia from caucasian clover have any effect on symbiotic N 2 fixation of white clover in pastoral situations.

Caucasian clover rhizobia are not a threat to nitrogen fixation by white clover

“…The presence of 50% or 99% caucasian clover rhizobia in the soil had no significant effect (P>0.05) on white clover forage DM (Figure 4A), %N in the forage or calculated symbiotic N 2 fixation (Figure 4B) in either year. This result conflicts with the glasshouse results of Pryor et al (2004) who found that the presence of >92% of caucasian clover rhizobia in the population significantly reduced N 2 fixation in white clover. The lack of effect in the field trial can be explained by the small percentage of nodules formed on white clover by caucasian clover rhizobia.…”

“…Elliot et al (1998) drew attention to the possibility that the use of caucasian clover, and the associated build up of it's rhizobia in the soil, could lead to a reduction of symbiotic N 2 fixation by white clover. This is supported by results from glasshouse trials (Pryor et al 2004) where N 2 fixation in white clover was significantly reduced when ineffective caucasian clover rhizobia made up a high proportion (>92%) of rhizobia in the growth medium. These glasshouse results suggested that the use of the present strains of rhizobia for caucasian clover inoculants, and the possible build up of ineffective rhizobia in soil, may over time reduce N 2 fixation and hence productivity of white clover based pastures.…”

“…These results clearly indicate that the effective rhizobia from white clover are more competitive at forming nodules on white clover than the ineffective rhizobia from caucasian clover under field conditions. Reasons for the marked difference in competitiveness of caucasian clover rhizobia between a previous glasshouse experiment (Pryor et al 2004) and this field trial are unclear. However, environmental conditions (Thies et al 1992) and different growth media (Singer et al 1964) have been shown to determine which components of the rhizobial population dominate in forming nodules.…”

“…Caucasian clover rhizobia numbers were higher (P<0.05) than the numbers of white clover rhizobia when there was more caucasian clover than white clover present in the tussock grassland swards. However, as pointed out by Pryor et al (2004) it is difficult to determine accurately the number of white clover rhizobia in soil when they are less than the number of caucasian clover rhizobia. Caucasian clover rhizobia readily form ineffective nodules on white clover.…”

Host-specific regulation of symbiotic nitrogen fixation in Rhizobium leguminosarum biovar trifolii

“…This effect is of potential agronomic importance in New Zealand, where pastoral agriculture is largely based on WC/ryegrass pastures. Widespread use of CC will likely lead to the build-up of significant populations of CC rhizobia in soils with the potential to adversely affect WC growth (Elliot et al, 1998;Pryor et al, 2004).…”